1In a small bowl with an electric mixer, beat the butter, zest, and sugar on medium speed until light and fluffy, scraping down the sides of the bowl as needed, about 2 minutes. Add the brandy and vanilla and beat until just smooth and incorporated, about 30 seconds.
2Scrape into a serving dish or plastic container, cover, and refrigerate overnight to meld the flavors. The sauce will have a solid consistency. Before serving, grate fresh nutmeg over the top. To serve, scoop out a small spoonful and push the sauce out of the spoon using a butter knife.